The present invention relates to an electrophotographic type image forming device such as a laser printer, a facsimile machine, and a copying machine, and more particularly, to the image forming device of so called cleanerless type in which residual toners after transfer operation is re-used in a subsequent developing operation instead of accumulation of the residual toners in a form of waste toners. The present invention also relates to a developing roller for use in the image forming device.
In an image forming device such as a laser printer and a copying machine, residual toners remaining on a photosensitive member after the transfer operation are scraped off by a resilient blade, and are stored in a waste toner case, and the case must be periodically scrapped.
However, in such a conventional image forming device, operator's hand or clothes are contaminated at the time of scrapping the toners, and toners may be dispersed during transportation of the waste toner case due to destruction of the toner case. Therefore, working environment may be polluted, and such defect does not meet with the ecological standpoint.
Therefore, so called cleaner-less system has been proposed in which residual toners remaining on the photosensitive member are collected by the developing means and is reused in a subsequent developing operation.
One example of a cleaner-less type image forming device will be described with reference to FIG. 16.
A laser printer 200 shown in FIG. 16 generally includes a main case 202, a sheet cassette 214 provided at a bottom of the main case 202, a sheet supply roller 213 for supplying each one of a sheet P stored in the sheet cassette 214, a photosensitive drum 220 provided above the sheet supply roller 213, a transfer roller 260 in pressure contact with the photosensitive drum 220 and is positioned therebelow, a fixing unit 270 disposed in the vicinity of the photosensitive drum 220, and a sheet discharge tray 277 disposed above the photosensitive drum 220. That is, a meandered or zigzag sheet transport pathway PP is provided starting from the sheet cassette 214 as shown by a dotted chain line so as to provide a compact laser printer 200.
After the electrical charge on the photosensitive drum 220 is removed by a discharge lamp 241, a laser beam L emitted from a laser scanner unit 230 is irradiated on the photosensitive drum 220 to form an electrostatic latent image in a state where a charger 240 uniformly charges the surface of the photosensitive drum 220 with a predetermined polarity. This electrostatic latent image is developed into a visible image by charged toners supplied from a developing roller 256 which is integrally disposed in a toner cartridge 250 and in contact with the photosensitive drum 220, thereby forming a toner image. This toner image is then transferred by the transfer roller 260 onto a fed sheet P. The toner image on the sheet P is fixed to the sheet P by a heat roller 271 of the fixing unit 270. Finally, the sheet P carrying thereon the output image is discharged onto the discharge tray 277.
On the other hand, residual toners remaining on the photosensitive drum 220 after the image transfer operation are temporarily stored on a cleaning roller 280 by applying to the cleaning roller 280 a voltage whose polarity is opposite the polarity of the toners. After completion of the transferring operation onto the sheet P, voltage having the polarity the same as that of the toners is then applied to the cleaning roller 280, so that the residual toners on the cleaning roller 280 are again discharged onto the photosensitive drum 220, and are collected into the toner cartridge 250 by the developing roller 256.
Here, in the toner cartridge 250, the developing roller 256 and a toner supply roller 255 in pressure contact with the developing roller for supplying toners 253 to the developing roller 256 are both rotatably disposed. Further, in the toner cartridge 250, a thickness regulation blade 257 is provided for regulating a thickness of the toner layer adhered onto the developing roller 256 into a predetermined thickness and for charging the toners 253. The developing roller 256 and the photosensitive drum 220 are driven to be rotated in directions shown by arrows so that surplus toners 253 removed by the thickness regulation blade 257 can be dropped onto the toner supply roller 255.
That is, the developing roller 156 is driven to be rotated in a direction opposite the rotating direction of the photosensitive drum 220 in pressure contact therewith, because the developing operation is performed while charging the toners 253 with the predetermined polarity by the developing roller. In other words, at a nip portion between the developing roller 256 and the photosensitive drum 220, these are driven to be moved in the same direction with each other.
Further, in the laser printer 200 shown in FIG. 9, the toners 253 are polymerized toners which provide high fluidity. If pulverized toners are used instead of the polymerized toners, amount of residual toners remaining on the photosensitive drum 220 may be increased after transfer operation due to low fluidity. Further, efficiency for collecting the residual toners by the cleaning roller 280 and the developing roller 256 may be lowered. As a result, residual toners can not be sufficiency collected, and may reach the transferring region, and are transferred onto the sheet P by the transfer roller 260. Consequently, a ghost image corresponding to the distribution of the residual toners on the photosensitive drum 220 may be generated in the sheet P.
Generally, polymerized toners and pulverized toners respectively provide advantage and disadvantage, and determination cannot be unqualifiedly made as to which one is superior to the other. The pulverized toners are provided by mulling wax, CCA(charge control agent), and coloring agent (carbon black if black color is to be generated) with a resin such as styrene acrylic resin and polyester resin, and then by pulverizing the mulled mixture. Due to the pulverization, toners may have irregular shape and have many corners. As a result, the toners do not provide sufficient fluidity. Accordingly, the toners do not provide sufficient spreading characteristic in the developing unit, to lower transferring efficiency. On the other hand, because of the low fluidity, toner leaking amount through a minute gap can be reduced, which facilitates a design of a sealing member which is adapted for avoiding toner leakage.
Constituents of the polymerized toners are not greatly different from those of the pulverized toners. However, method for producing the polymerized toners is quite different from the method for producing the pulverized toners. For producing the polymerized toners, raw materials are dispersed in a solvent, so that the polymerized toner has generally spherical shape because of the surface tension. According to the recent technique, it is impossible to produce toners by polymerization method using polyester resin. Therefore, styrene acrylic resin is widely used. The polymerized toner has generally spherical shape, so that it can provide high fluidity. As a result, the toners are sufficiently spread in the developing unit to provide high transferring efficiency. However, due to the high fluidity, toners may be easily leaked through a fine gap, and therefore careful attention is required in designing a sealing member.
Further, in the laser printer 200, as shown by the dotted chain line in FIG. 9, the sheet transport pathway PP is meandered, and therefore, a special sheet such as a thick sheet or an envelope are not available which are disadvantageous for image transfer. If a linear sheet passage can be provided, printing can be made on the thick sheet and the envelope. However, in this case, amount of residual toners remaining on the photosensitive drum 220 may be increased due to insufficient transfer of the toners. Accordingly, the residual toners cannot be completely collected by the cleaning roller 280 and the developing roller 256, so that ghost may be generated on the sheet P, similar to the above described disadvantages.
That is, as described above, it is difficult in the cleaner-less type image forming device to use the pulverized tones which are easily produced but provides low fluidity. Therefore, the device must use the polymerized toners which provide high transferring efficiency but are difficult to produce. Consequently, design of a sealing member may become difficult, the sealing member being adapted for preventing the toners from leakage at a position around the developing roller 256.
Further, in using the polymerized toners, amount of residual toners remaining on the photosensitive drum may be increased particularly in a low temperature, low humidity condition when image is formed on the special sheet such as the thick sheet and the envelope those providing low transferring efficiency. Therefore, the residual toners cannot be completely collected by the developing roller, and ghost may be generated.
The cleaner-less type still requires means for temporarily storing residual toners during printing such as the above described cleaning roller 280. Therefore, additional power source is required in comparison with a system where cleaning blade is provided for scraping off the residual toners.
Further, instead of the cleaning roller, toner leveling brush can be used for leveling the residual toners on the photosensitive drum. In this case, additional power source is unnecessary. However, the toner leveling brush cannot completely collect the residual toners if the amount of the residual toners is increased, and therefore, ghost may be generated.
Further, in another conventional electrophotographic type image forming device, transferring means such as a transfer roller is disposed above the photosensitive drum. This structure is advantageous in that accessing to a sheet passage becomes possible by opening the device from upper side so as to deal with the sheet jamming. This is advantageous over a structure shown in FIG. 16 in which the transfer roller 260 is disposed below the photosensitive drum 220. Further, if the transfer roller is disposed above the photosensitive drum, a sheet cassette can be exposed to the upper side, which facilitates supply of the sheet, and furthermore, a linear sheet passage can be provided, which is capable of printing to a special sheets such as a thick sheet, an envelope and OHP sheet. With such reasons, in the conventional image forming device, the transfer roller is desired to be disposed above the photosensitive drum.
On the other hand, according to the developing process in this type of image forming device, an elastic roller formed of an electrically conductive silicone is used as the developing roller which carries toners, and so called impression developing process is performed in which the developing roller is depressed against the photosensitive drum for developing operation. According to this process, efficient developing operation can be performed by enhancing the developing efficiency of the toners. However, the toners also serve as lubricants. By the existence of the toners at the nip portion where the developing roller and the photosensitive drum are in contact with each other, developing operation can be performed with maintaining a smooth contact between the developing roller and the photosensitive drum.
However, as described above, in the impression developing method in which the transfer roller is disposed above the photosensitive drum, rotating direction of the developing roller is opposite to the rotating direction of the photosensitive drum with respect to the nip portion taking the sheet feeding direction(rotating direction of the photosensitive drum) and toner supplying direction toward the photosensitive drum into consideration. Therefore, if developing operation is performed with the developing efficiency close to 100% in case of the solid black printing, almost all the toners are used for the developing the electrostatic latent image on the photosensitive drum at a position above the nip portion. Accordingly at the nip portion, extremely small volume of toner or no toner is provided. As a result, smooth contact between the developing roller and the photosensitive drum at the nip portion cannot be maintained due to non-existence of the lubricant, i.e., the toners. As a result, a surface of the developing roller may be irregularly deformed or vibrated to cause chattering due to the friction between the developing roller and the photosensitive drum. Finally, corrugated pattern may be generated in an output image due to the chattering. On the other hand, by lowering the developing efficiency, it is possible to retain the toners at the nip portion on the developing roller even in case of the solid black printing. However, this leads to degradation of the output image.
Further, in the impression developing method in which the transfer roller is disposed above the photosensitive drum and rotating direction of the developing roller is opposite to the rotating direction of the photosensitive drum with respect to the nip portion, the developing roller formed of silicone may easily be worn, so that striped scratches may be generated on the developing roller, which leads to insufficient printing where striped pattern is formed in an output image. Further, service life of the developing roller, which is expendables, may be shortened.
Further, in the electrophotographic type image forming device, in order to enhance imaging quality, fluidity is required in the toners so that the toners can be properly subjected to developing, transferring and cleaning on the photosensitive drum. To this effect, the surface of the toner is added with additives such as silica.
If the toners having high fluidity are used capable of performing high imaging quality (for example, 600 dpi), the toners scraped from the developing roller by the photosensitive drum is deposited on the upper portion of the nip portion due to the rotational directions of the photosensitive drum and the developing roller. As a result, the deposited tones may adhere to an unwanted portion due to the Van der Waals attraction on the photosensitive drum, which portion does not correspond to the electrostatic latent image portion. As a result, foggy portion may be generated on the transferred image in the sheet, to lower the quality of the image.
In another aspect, the developing roller is imparted with an electrical conductivity by an electron conduction capable of being applied with the developing bias voltage. That is, electrical conductivity is imparted on the developing roller by dispersing electrically conductive minute particles such as carbon particles into the elastic material which is the base material of the developing roller.
In this developing method, efficient developing can be performed by enhancing developing efficiency of the toners. On the other hand, the toners also serve as electrically insulating materials located between the photosensitive drum and the developing roller to which the developing bias is applied. Because of the existence of the toners at the nip portion between the developing roller and the photosensitive drum, surface voltage level of the photosensitive drum can be stably maintained in developing operation.
However, as described above, if 100% developing efficiency is contemplated in the developing operation, almost all the toners are adhered to the photosensitive drum at a position above the nip portion in case of solid black printing. Accordingly at the nip portion, extremely small volume of toner or no toner is provided serving also as the electrical insulator. As a result, electrical insulation cannot be maintained between the developing roller and the photosensitive drum at the nip portion due to non-existence of the toner, and consequently, the developing bias voltage applied to the developing roller may be leaked to the photosensitive drum. Accordingly, the surface voltage of the photosensitive drum becomes unstable, to lower imaging quality.
On the other hand, it is possible to still retain toners at the nip portion on the developing roller even in the case of solid black printing by lowering the developing efficiency. However, this causes degradation of the output image.